Issue #3048134 by Pancho: Properly document $context arrays in FillPdfLinkManipulatorInterface::parseLink()

A few things happened here. I split out the access-checking logic into a
FillPdfAccessHelper service. In coordination with that, I added
FillPdfLinkManipulator::parseUrlString() to facilitate parsing the
context strings we store in FillPdfFileContext objects. I also changed
the way I store URLs in FillPdfFileContext.context so that they are
easier to pass to Url::fromUri() for parsing (and I made a helper
method that parses them into a form Uri::fromUri() likes).

In short, saving PDFs to files now works as well as downloading. Score!

And a lot more. Fleshed out some services.

The saving part is almost working. Need to get FillPDF file contexts
being saved properly. Then, I need to ensure that access
checking on the files works like I expect. I will probably have to
implement some hook or another.